Dynamically restrict accesses to virtual hosts' documentroots

I have a lot of virtual hosts. They are made with rewrite-engine so they do not each have a <VirtualHost> section. I would like for PHP to restrict access from PHP to $DOCUMENT_ROOT. Also I would like to have / mean $DOCUMENT_ROOT. This means that if http://www.example.com/foo/bar.php opens a file called "/fubar.html" then this file will be the same as: - http://www.example.com/fubar.html - $DOCUMENT_ROOT/fubar.html - /home/of/www.example.com/public_html/fubar.html www.example.net lives on the same machine. If http://www.example.net/baz/xyzzy.php opens a file called "/quux/quuux.txt" then this file will be the same as: - http://www.example.net/quux/quuux.txt - $DOCUMENT_ROOT/quux/quuux.txt - /home/of/www.example.net/public_html/quux/quuux.txt What I am looking for is something that smells like chroot without actually being it. It should be so dynamically because it will be impossible to make all the <VirtualHost>-statements as VirtualHosts are generated dynamically with the rewrite-engine. I have looked into doc_root and it seems to do part of the trick - though only statically. What I need is dynamically. I am looking for a way to say: doc_root = $DOCUMENT_ROOT (Calculated for each request. Not at boot time) and prepend $DOCUMENT_ROOT to the file name for each file access that starts with '/'. I am sure others that host a lot of virtual hosts have the same problem, but I have not been able to find the solution. /Ole